1. Field of the Invention
The invention relates to optical-scanning systems and, more particularly, to a method and apparatus for the direct recording of electrical signals corresponding to a scanned image onto a moving recording medium.
2. Description of the Prior Art
Considerable effort has been expended over at least the last decade towards converting an optical image to an electrically processible equivalent through the use of photodiode arrays. Because of limitations in semiconductor chip area, the available photodiode arrays are either square arrays of relative low resolution, or linear arrays of relatively high resolution. Various mechanisms have been developed to move linear arrays perpendicular to their axes so that they can provide area coverage similar to the square arrays. U.S. Pat. Nos. 4,149,091 (Crean et al.) and 3,867,569 (Watson) are descriptive of such mechanisms.
The above patents transmit the electrical information developed. U.S. Pat. No. 4,131,919 (Lloyd et al.) discloses apparatus for storing the electrical information. The method involves digitizing the information in each pixel, doing short-term electronic buffer storage, and then long-term magnetic storage. This is one approach to resolving the problem of the difference between the relative data rates of the information generator and the final storage media. Lloyd et al. are forced thereby to do the storage operation twice. While this duplication of effort is possible with the relatively small amount of information developed by a square array, the amount of information generated by a mechanically transported linear array makes such intermediate storage essentially impossible.
It is of interest to go through a numerical example to demonstrate the magnitude of the problem. It is possible to fabricate 1,600 photodiodes on a block of silicon. These could be arranged in a 40-point.times.40-point square array. If the output of each diode were digitized onto 8 bits, the intermediate buffer would have to store 12,800 bits. If the photodiodes were organized into a single line and mechanically transported to observe a square area 1,600 points by 1,600 points, there would be a 2.56.times.10.sup.6 pixels or 2.times.10.sup.7 bits of information generated.
An electronic camera that can be moved around easily has an additional value over those that are so bulky as to be immobile. The intermediate storage requirements of a mechanically transported linear array, whose information is processed using the method disclosed by Lloyd et al., would result in a rather hefty system regardless of the nature of the scanning mechanism.